1. Field of the Invention
This invention relates to a method and an apparatus for identifying an optical transmission medium and to an apparatus for identifying the same as well as to a method for optical telecommunication.
2. Description of the Prior Art
In the course of recent developments in telecommunication networks, a large number and a wide variety of telecommunication cables have been and are being laid in tunnels, conduits and ducts in building sites often in the form of bundles.
Such installations normally also contain power transmission cables whose external view resembles that of telecommunication cables.
When, for example, a specific cable has to be branched at a point somewhere in the middle thereof, the cable has to be identified in the first place at the point of branching.
Although the process of identification may seem very simple since a cable can be identified by introducing light from an end of the cable and detecting the light coming out from its other end, an operator for the branching operation positioned at the point of branching somewhere in the middle of the cable can not simultaneously see the incoming light end and the outgoing light and of the cable particularly when the cable is considerably long.
As far as metal cables for power transmission are concerned, there has been proposed a method for identifying a particular cable out of a bundle of cables by using a search coil.
With this method of identification, the magnetic field generated outside of a live metal cable is detected by a coil.
This technique can be advantageously used as it is a noncontact method for singling out a live metal cable out of a bundle of cables installed together.
However, the above technique is effective only in identification of a metal cable and it cannot be applied for identifying an optical fiber cable (nonmetal cable) that does not generate a magnetic field from without.
Thus there has been proposed, for identification of coated optical fibers, a so-called local detection method, which will be described below.
Each of the coated optical fibers, including the one to be detected, can be identified at its ends.
Therefore, in the local detection method, a given optical signal is introduced at an end of the optical fiber cable into the coated optical fiber which is assumed to be the right one for branching at a point in the middle of its longitudinal direction and then each of the coated optical fibers within the cable sheath is partially exposed and bent at the point.
Under these conditions, the optical fiber into which light has been introduced leaks the light (in a radiation mode) from the bent area. So, the optical fiber in question can be identified by detecting the source of the leaked light.
A telephone line is often used on sites of installing or repairing optical fiber cables for communication between workers.
When an optical fiber cable contains metal wires, any such wires may be utilized for telephone communication.
However, if the optical fiber cable to be laid is a nonmetallic cable that does not contain any metallic wires or if the optical fiber cable contains metallic wires but any of them can not be suitably used for telephone communication, a coated optical fiber is used as a telephone line, to which optical telephone sets are connected.
While the above described local detection method may be used when the cable sheath of an optical fiber cable can be partially peeled off and a particular optical fiber can be detected out of a number of optical fibers contained in it, the method may not be successfully used particularly when a large number of optical fiber cables are laid in parallel and a particular optical fiber can not be detected without peeling off all the sheath of the cables.
Besides, the process of peeling off, if partially, the sheath of an optical fiber cable in this method entails damage to the cable and that of bending the optical fiber cable can be accompanied by possible deterioration of the transmission characteristics of the cable, which in turn results in bit errors particularly when the cable is in live condition and actually being used.
The light signals for telephone communication are transmitted from and received at the ends of the coated optical fiber by the optical telephone sets.
While the optical telephone sets have to be connected to both ends of the coated optical fiber for telephone communication, such connection can not be realized when there are no cuttable coated optical fibers or when all the work for connection of the optical fiber cable has been completed.
If optical telephone communication has to be established under these circumstances by using one of the coated optical fibers of the cable, the selected fiber should be cut and optical telephone sets should be connected to the fiber at both ends, entailing a troublesome operation to be performed.